A dry processing method, such as a carding method or an airlaid method, is generally used to obtain a bulky non-woven fabric. Although the dry method allows to easily obtain a bulky non-woven fabric by providing crimps of various shapes, significant dispersion irregularity occurs in the mass per unit area and fibers, thus it is difficult to use the dry processing method for the purpose of obtaining a high uniformity. When applied to, for example, a battery separator, significant dispersion irregularity of the mass per unit area or fibers of a non-woven fabric to be used causes a short circuit and leakage of the electrolyte solution. In application of a high-efficiency filter, irregularity in a flow rate in a thin section may be caused, and in application of a cataplasm material, leakage of the chemical and the like may be caused.
Moreover, it is known that although a synthetic fiber such as a conjugate fiber can produce high non-woven fabric strength by forming a bulky web thereof into a non-woven fabric through heat processing, flattening of the fiber component is caused by heat-melting it, and the degree of freedom is controlled by adhering the fiber component with other fiber, reducing bulkiness.
On the other hand, a wet paper-making method developed out of an ancient paper pressing technology, and not only natural fibers such as pulp but also synthetic fibers or synthetic pulp are currently used in relatively large numbers since they can be supplied suitably at low cost. The wet paper-making method evenly disperses these fibrous matters in water and then cards the fibrous matter to thereby produce various characteristics, whereby a paper having high uniformity in the mass per unit area and thickness (a non-woven fabric obtained through a wet paper-making method) is obtained. The wet paper-making method is applied to a wide range or areas, such as sliding-screen papers, moist towelettes and the like for general purposes, and, for high-function purposes, a high-efficiency filter required to have a uniform film-thickness and a battery separator required to have high liquid-retaining ability associated with film thickness.
Most of the fibrous matters of a paper include functional synthetic fibers in order to provide the strength of the paper or a value-added characteristic. In order to have improved dispersibility in water, straight short fibers are often used as the synthetic fibers so that the fibers are dispersed easily without entangling with each other. As a result, thus obtained paper is in the form of thin paper reflecting low bulkiness of the straight fibers. Therefore, the wet paper-making method is considered unsuitable as a process for obtaining a bulky non-woven fabric.
In order to solve such problems, for example, in Japanese Patent Application Publication (hereunder referred to as “JP KOKAI”) No. Sho 62-268900, there is proposed a method of blending highly stiff inorganic fibers, especially glass fibers, in order to improve liquid-retaining ability of a paper used in a batter separator. This secures a gap for retaining liquid, because it has a constant bulkiness and rigidity while forming a dense matrix by means of fine glass fibers. Also, for example, in JP KOKAI No. 2001-32139, there is proposed a method of producing a non-woven fabric using only latently crimping fibers, wherein three-dimensional crimping is produced in synthetic fibers by thermally shrinking them to provide bulkiness. However, the method using glass fibers is not exactly a suitable method because, although it can obtain bulkiness, extremely high cost is incurred and glass fiber is a material imposing an environmental load because it cannot be disposed or incinerated easily. Furthermore, the method using only latently crimping fibers is not exactly a suitable method due to its operational performance in which the production dimension is unstable and mass per unit area irregularity occurs easily, since bulkiness is produced by contracting the fibers. Moreover, it is necessary to introduce a processing device in which fibers can have an appropriate degree of freedom so as to be able to move at the time of contraction, but it is inevitable that investment on such a device is disadvantageous in view of cost.
Therefore, it is extremely difficult to obtain a bulky non-woven fabric while maintaining uniform dispersion of mass per unit area and fibers.